sigma

Meaning: A sigma (σ) bond forms when orbitals overlap end-to-end, so the electron density sits on the axis between the nuclei. Overlaps of s–s, s–p, p–p (head-on) and hybrid orbitals (sp, sp², sp³) all give sigma bonds. A single bond is one sigma bond, a double bond is one sigma plus one pi (π), and a triple bond is one sigma plus two pi. Pi bonds come from side-by-side overlap of unhybridized p orbitals, which is less effective, so a pi bond is typically weaker than the sigma bond between the same atoms.

On the MCAT: You may be asked to count bonds in a drawn structure. Ethene (C₂H₄) has 5 sigma and 1 pi; ethyne (C₂H₂) has 3 sigma and 2 pi. Steric number = sigma bonds + lone pairs on an atom, and it gives hybridization (4 = sp³, 3 = sp², 2 = sp) and electron geometry. Also expect rotation questions: atoms can rotate about a sigma single bond (giving conformers) unless a ring restricts it, while a pi bond blocks rotation and makes cis/trans isomers possible. When a double bond breaks, the pi component goes first.

Common confusion: A double bond is not two sigma bonds; only one of its two bonds is sigma. Lone pairs count toward steric number but are not sigma bonds. In statistics, σ means standard deviation.